The present invention generally pertains to frequency synthesizers and is particularly directed to an improvement in frequency-agile, mulit-channel frequency synthesizers.
Prior art frequency synthesizers typically utilize a phase-locked loop which produces a loop output signal at a selected synthesizer output frequency in response to a phase comparison of a signal derived from a sample of the loop output signal and a signal derived from a reference signal having a predetermined frequency. In various different prior art frequency synthesizers one or both of the signals that is compared is derived by frequency dividing by one of a given number of different predetermined factors, whereby the synthesizer output frequency is varied by varying the selected frequency-division factor. In a typical prior art frequency synthesizer, the frequency division factor is provided by a frequency divider within the phase-locked loop. Typical prior art frequency synthesizers utilizing a phase-locked loop are described in U.S. Pat. Nos. 4,360,788 to Erps et al; 4,249,138 to Bell; 4,234,929 to Riley, Jr.; 4,225,828 to Watanabe et al; 4,156,205 to Kadin et al; 4,105,948 to Wolkstein; 4,105,946 to Ikeda; 4,061,973 to Reimers et al; 3,676,794 to Bidell et al; 3,611,175 to Boelke; and 3,401,353 to Hughes. Some of the various systems described therein include such techniques for deriving different ones of the compared signals as mixing the sample signal with a selected variable frequency control signal (Erps et al); and mixing sample signal with a large number of harmonics of the reference signal (Kadin et al). Wolkstein describes summing and analog range selection voltage signal with the comparison signal and providing the resultant signal to control a voltage-controlled oscillator in the phase-locked loop; processing an input signal identifying a selected synthesizer output frequency to provide both a range select signal from which the analog rang selection voltage signal is derived and a signal indicating a selected frequency-division factor.
It is the object of the present invention to provide an improved multi-channel frequency synthesizer that is capable of providing a large number of frequency channels with great agility for use as a local oscillator in frequency-hopping application, and yet which is simple and reliable in construction and operation.